Method for the reversible storage of hydrogen

ABSTRACT

A process for the reversible storage of hydrogen, characterized in that the complex alkali metal aluminium hydrides (alkali metal alanates) of general formula 1 ##EQU1## are used as the reversible hydrogen storage materials.

This application is a 371 of PCT/EP96/03076, which was filed on Jul. 12, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the reversible storage of hydrogen in the form of complex alkali metal aluminium hydrides (alkali metal alanates).

2. Description of Related Art

The methods for the storage of hydrogen used today in the art are predominantly the storage as a compressed gas in pressure tanks, at normal pressure in gasometers, and at low temperatures (≦20 K) as liquid hydrogen.

SUMMARY OF THE INVENTION

A more recent method for hydrogen storage (H₂ storage) which is currently being developed, especially for the use of hydrogen as a fuel (combustible), is based on the reversible thermal dissociation of metal hydrides (MH_(n), Equation 1; H. Buchner, "Energie-speicherung in Metallhydriden", Springer-Verlag 1982; G. Sandrock et al., in "Hydrogen in Intermetallic Compounds II", page 197 (Ed. L. Schlapbach), Springer-Verlag 1992). In addition to H₂ storage for stationary or mobile use, reversible metal hydride/metal systems (Equation 1) can be used technically for a number of other, potential or already realized, applications, such as hydrogen separation, purification and compression, heat storage, heat conversion and refrigeration (heat pumps), and as electrodes for electric batteries. ##EQU2## M=metal, metal alloy, intermetallic compound

The reversible H₂ storage in the form of metal hydrides has several advantages over conventional storage methods. Over compressed H₂ gas, metal hydrides have considerable advantages with respect to the achievable volumetric storage density. In addition, metal hydrides have the advantage, with respect to safety, that their hydrogen dissociation pressure is lower by powers of ten as compared to the same concentration of pressurized hydrogen. The volumetric H₂ densities achievable with hydride containers reach those of liquid hydrogen containers without the necessity of using cryotechnology, which is expensive and cumbersome. The disadvantages of the latter can be seen, inter alia, from the fact that the recovery of one unit of energy of liquid hydrogen requires 2.5 to 5 times as high an expense of primary energy.

The main drawback of the currently known reversible metal hydrides as H₂ storage materials, as compared to liquid hydrogen, is their relatively low storage density per weight of storage material (expressed in % by weight of H₂ in the metal hydride) Magnesium hydride (MgH₂, 7.6% by weight of H₂) and hydrides of magnesium alloys (Mg₂ NiH₄, 3.7% by weight of H₂) can compete technically with liquid hydrogen in this respect, provided that enough heat above 300° C. is available for desorption of the hydrogen from the hydride.

The most serious disadvantage of the so-called low and medium temperature hydrides known today (H. Buchner, 1982, pages 26-29) is the high costs of the intermetallic compounds and alloys used for H₂ storage while their H₂ storage capacity is lower by a factor of 4-5 than that of MgH₂ (LaNi₅ : 1.4%; TiFe: 1.9% by weight of H₂). From this point of view, it appears highly desirable and technically necessary to develop novel reversible low and/or medium temperature metal hydrides with higher H₂ storage capacities than are known to date (Sandrock 1992, page 220; S. Suda, G. Sandrock, Ztschr. Physikal. Chem., Neue Folge 1994, 183, 149).

It has now been surprisingly found that the complex sodium and potassium alanates and the mixed sodium-lithium, sodium-potassium and potassium-lithium alanates of general formula 1 are suitable as reversible H₂ storage materials under certain conditions. In addition, it has been found that the properties of compounds 1 as reversible H₂ storage materials can be still improved considerably by doping with foreign metals, intermetals and their hydrides according to the invention.

    M.sub.p(1-x).sup.1 M.sup.2.sub.px AlH.sub.3+p              (1)

    M.sup.1 =Na, K M.sup.2 =Li, K 0≦x≦˜0.8 1≦p≦3

Sodium alanate, NaAlH₄, is produced on a technical scale. Na₃ AlH₆ can be prepared from NaAlH₄ and NaH in the presence of hydrogen (Equation 2) (L. Zakharkin, V. Gavrilenko, Dokl. Akad. Nauk SSSR 1962, 145, 793, Engl. Vol. 145, 656).

    NaAlH.sub.4 +2NaH→Na.sub.3 AlH.sub.6                (2)

The mixed alanate Na₂ LiAlH₆, as yet unknown, was synthesized under hydrogen pressure according to Equation 3.

    NaAlH.sub.4 +NaH+LiH→Na.sub.2 LiAlH.sub.6           (3)

From the literature (E.Ashby, P. Kobetz, Inorg. Chem. 1966, 5, 1615; T. Dymova et al., Dokl. Akad. Nauk SSSR 1975, 224, 591, Engl. 556), it is known that the thermal dissociation of solid NaAlH₄ takes place in two steps: in the first step, NaAlH₄ decays to Na₃ AlH₆ and metallic aluminum with release of hydrogen (Equation 4); then, at higher temperatures, there is again release of hydrogen from Na₃ AlH₆ to form NaH and Al (Equation 5). The overall course of the thermolysis of NaAlH₄ is represented in Equation 6. (The dissociation of NaH to Na and hydrogen takes place only at considerably higher temperatures.)

    NaAlH.sub.4 →1/2Na.sub.3 lH.sub.6 +2/3Al+H.sub.2    (4)

    1/2Na.sub.3 AlH.sub.6 +2/3Al →NaH+Al+1/2H.sub.2     (5)

    NaAlH.sub.4 →NaH+Al+3/2H.sub.2                      (6)

In contrast, the thermolysis of Na₃ AlH₆ takes place in one step according to Equation 7.

    Na.sub.3 AlH.sub.6 →3NaH+Al+3/2H.sub.2              (7)

Although the thermal dissociation of NaAlH₄ and Na₃ AlH₆ to NaH, Al and hydrogen (Equations 6 and 7) has been described and the related H₂ dissociation pressures experimentally determined (Dymova et al., 1975), the reversibility of this reaction apparently has not been recognized to date. Thus, the decomposition of NaAlH₄ to Na₃ AlH₆ and of the latter to NaH and Al is said to be "irreversible" (Dymova et al., 1975, page 557: " . . . the irreversible decomposition of NaAlH₄ leads to Na₃ AlH₆ which, in its turn, decomposes to NaH."). That the reactions of Equations 6 and 7 are believed to be irreversible can also be seen from the cited article because of the fact that the H₂ dissociation pressures have only been measured in the direction of H₂ desorption (cf. the text on page 5). In an earlier work by the same group (T. Dymova et al., Dokl. Akad. Nauk SSSR 1974, 215, 1369, Engl. 256, "Direct Synthesis of Alkali Metal Aluminium Hydrides in the Melt"), there is reported, inter alia, a direct synthesis of sodium alanate (NaAlH₄) from Na, Al and hydrogen in the molten state (Equation 8) at temperatures below 270-280° C. and pressures above 175 bar. From these references, it can be seen that the reaction mixture is present in liquid form under the conditions of synthesis which should enable an intimate contact between the reactants. Since sodium hydride (NaH) will decompose at about 420° C. without first melting, a synthesis of NaAlH₄ from NaH (solid), Al (solid) and H₂ is not to be expected from the literature cited.

    Na(liquid)+Al(solid)+2H.sub.2 →NaAlH.sub.4 (liquid) (8)

mp. 97.8° C. mp. 187° C.

Therefore, from the prior art, it could not be foreseen or expected that NaAlH₄ or Na₃ AlH₆ could be used as reversible H₂ storage materials. However, it has been surprisingly found that the NaH/Al mixtures obtained in active form after the thermolysis of NaAlH₄ or Na₃ AlH₆ (Equations 6 and 7) are rehydrogenated to NaAlH₄ or Na₃ AlH₆, respectively, under certain conditions (Examples 1 and 4). Since the process of thermolysis of sodium alanates with the release of hydrogen and their renewed synthesis with the uptake of hydrogen can be repeated, it is possible to use the sodium alanate/(NaH+Al) systems as reversible H₂ storage systems. These are the first known hydrogen storage systems which are based on the reversible reactions of the solid mixtures of a metal hydride (NaH) and a metal (Al) with hydrogen (Equations 9 and 10). To a different extent, this also applies to other alkali metal alanates as defined according to formula 1. ##EQU3##

Another inventive feature of the present process is the fact that the process of hydrogen release and uptake by alkali metal alanates can be accelerated or made to proceed more completely by the addition of catalysts. For catalyzing the hydrogen de-charging and charging reactions (H₂ desorption and H₂ adsorption, respectively), the reversible alkali metal alanates 1 are doped with foreign metal compounds according to the invention. For such doping, alkali metal alanates are reacted or mechanically stirred with foreign metal compounds in an organic solvent or without a solvent. Suitable dopants are compounds of the transition metals of groups three to five of the periodic table (Sc, Y, Ti, Zr, Hf, V, Nb, Ta) as well as compounds of iron, nickel and the rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy; Ho, Er, Tm, Yb, Lu). Preferred dopants are alcoholates, halides, hydrides and organometallic and intermetallic compounds of the mentioned metals. Combinations thereof may also be employed. The dopants are employed in amounts of from 0.2 to 10 mole %, based on alkali metal alanates 1, preferably in amounts of from 1 to 5 mole %, based on 1. If the transition metals are present in a higher oxidation state, they are reduced to a low-valent oxidation state by the alkali metal alanates, which are present in excess, in the course of the doping process. The reduction process can be detected and quantified by the hydrogen evolution during the doping.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the drawings wherein:

FIG. 1 is a graph depicting H₂ desorption from Na₃ AlH₆ at normal pressure;

FIG. 2 is a graph depicting H₂ desorption from NaAlH₄ at normal pressure;

FIG. 3 is a graph depicting Na₃ AlH₆ cycle stability;

FIG. 4 is a graph depicting NaAlH₄ cycle stability;

FIG. 5 is a graph depicting hydrogen charging of dehydrogenated sodium alanate at 170°C.;

FIG. 6 is a graph depicting the experimentally established concentration-pressure isotherms of the NaAlH₄ /(NaH+Al) system doped with 2 mole % of Ti at 180°C. and 211°C.; and

FIG. 7 is a graph depicting the experimentally established concentration-pressure isotherms of the Na₃ AlH₆ /(3 NaH+Al) and Na₂ LiAlH₆ /(2 NaH+LiH+Al) system doped with 2 mole % of Ti at 211°C..

An important feature of metal hydrides as reversible H₂ storage materials, e.g., for mobile use, is the rate of hydrogen desorption at different operating temperatures. By the catalytic acceleration of the H₂ desorption, the temperature at which the desorption proceeds at a rate which is sufficiently high for technical applications can be considerably lowered. Thus, for example, FIG. 1 (Example 2) shows that undoped Na₃ AlH₆ will release hydrogen at a hardly remarkable rate at 160° C. Even at 200° C., the dehydrogenation is still relatively slow. In contrast, in Na₃ AlH₆ doped with 2 mole % of Ti, the desorption proceeds at a nearly constant rate at 160° C. and is virtually completed within 4-5 h already. This is similar with the H₂ desorption from undoped as compared to that of Ti doped NaAlH₄ (FIG. 2, Example 5).

The improvement of H₂ absorption performance of the reversible alkali metal alanate H₂ storage systems by foreign metal doping can be demonstrated by both the rate and the extent of H₂ absorption in a number of dehydrogenation/rehydrogenation cycles (cycle tests). The improvement in H₂ uptake of the Na₃ AlH₆ /(3 NaH +Al) system doped with 2 mole % of Ti in comparison with the corresponding undoped system under the given hydrogenation conditions is shown in FIG. 3 (Example 1). The reversible H₂ content of the Ti doped system is significantly higher than that of the undoped system; in addition, the Ti doped Na₃ AlH₆ shows a higher cycle stability as compared to the undoped material.

A dramatic increase in H₂ absorptivity of the reversible NaAlH₄ /(NaH+Al) system results from Ti doping, e.g., with 2 mole % of TiCl₃. In a typical cycle test (FIG. 4, Example 4), the reversible H₂ content of the doped sample is from 3.1 to 4.2% by weight while the undoped sample will store only from 0.5 to 0.8% by weight of hydrogen under the same hydrogenation conditions.

The improvement of the rate and extent of H₂ absorption of the reversible NaAlH₄ /(NaH+Al) system by Ti doping can be demonstrated particularly clearly by the hydrogenation curves in FIG. 5 (Example 5); as shown in the figure, the NaH+Al mixture obtained from the dehydrogenation of NaAlH₄ doped with Ti(OBu)₄ can be hydrogenated to NaAlH₄ at 170° C./152-125 bar substantially more rapidly than the TiCl₃ doped material. The degree of rehydrogenation after 15 h under these conditions is 3.9% by weight of H₂ with both Ti(OBu)₄ and TiCl₃ doping. Under the same hydrogenation conditions, a degree of rehydrogenation of only 0.8% by weight of H₂ is achieved with the undoped NaAlH₄ (Example 4).

The evaluation of the reversible metal hydride/metal systems with respect to their maximum achievable H₂ storage capacity as well as the conditions under which hydrogen charging and decharging is possible under principal (thermodynamic) aspects is generally performed by so-called concentration-pressure isotherms (cpi diagrams). The experimentally established cpi diagrams of the NaAlH₄ /(NaH+Al) system doped with 2 mole % of Ti (Example 4) at 180 and 211° C. are shown in FIG. 6, and those of the Ti doped Na₃ AlH₆ /(3 NaH+Al) and Na₂ LiAlH₆ /(2 NaH+LiH+Al) systems (Examples 1 and 3) at 211° C. are shown in FIG. 7. As shown in the Figures, the cpi diagrams of the hydride systems according to the invention could be established in the direction of both H₂ desorption and H₂ absorption, which furnishes evidence for their usefulness in reversible H₂ storage and disproves the assumption of irreversibility of the thermal decomposition of NaAlH₄ and Na₃ AlH₆ found in the cited literature (text on page 3).

In the cpi diagram of the NaAlH₄ /(NaH+Al) system (FIG. 6), two temperature-dependent pressure plateaus can be seen which correspond to the two-step dissociation of NaAlH₄ (Equations 4 and 5). In contrast, the cpi diagram of the Na₃ AlH6/(3 NaH+Al) system (FIG. 7) shows only one pressure plateau, in accordance with the one-step reversible dissociation of Na₃ AlH₆ (Equation 7). From the broadness of the pressure plateaus, it can be seen that the Ti doped NaAlH₄ /(NaH+Al) system (FIG. 6) disposes of a maximum achievable H₂ storage capacity of 3.2% by weight through the first dissociation step, of 1.7% by weight through the second, and of 4.9% by weight of H₂ through the two dissociation steps. In the cycle tests performed (FIG. 4, Example 3), storage capacities of up to 4.1% by weight of H₂ are achieved through the two dissociation steps, depending on the hydrogenation condition. The Ti doped Na₃ AlH₆ /(3 NaH+Al) system (FIG. 7) disposes of a maximum storage capacity of 2.7% by weight of H₂, and in cycle tests (FIG. 3, Example 1), up to 2.3% by weight of H₂ is achieved. Thus, the reversible NaAlH₄ /(NaH+Al) system is distinguished from the Na₃ AlH₆ /(3 NaH+Al) system by a substantially higher reversible H₂ storage capacity. This goes along with the drawback that the former system requires relatively high hydrogen pressures (e.g., 130-150 bar) for charging with hydrogen (e.g., at 170° C.; Example 4, FIG. 4), which is due to the high H₂ equilibrium pressure (FIG. 6). In contrast, it is characteristic of the Na₃ AlH₆ /(3 NaH+Al) system that charging with hydrogen can be done under substantially lower hydrogen pressures (e.g., 40-60 bar at 200° C.; Example 1, FIG. 3) due to the relatively low H₂ equilibrium pressure (FIG. 7; 32-34 bar at 211° C.).

The conditions for hydrogen charging and hydrogen decharging of the alkali metal alanate systems according to the invention (e.g., Equations 9 and 10) at a particular temperature are governed by the thermodynamically caused and experimentally determinable hydrogen equilibrium pressures (FIGS. 6 and 7). If the external H₂ pressure exceeds the hydrogen equilibrium pressure and the system is in an uncharged or partially charged condition, H₂ absorption occurs. Conversely, if the external H₂ pressure is lower than the hydrogen equilibrium pressure and the system is in a charged or partially charged condition, H₂ desorption occurs. For the rate of H₂ absorption or H₂ desorption to attain a finite value, the temperature at which the H₂ charging or H₂ decharging occurs must not be lower than ˜100° C. For hydrogen charging at a given temperature, external H₂ pressures of from 0.1 to 100 bar above the hydrogen equilibrium pressure, preferably from 2-3 to 50 bar above the hydrogen equilibrium pressure, are to be used. For hydrogen decharging, external H₂ pressures of from 0.1 bar below the hydrogen equilibrium pressure to 0.1 bar, preferably from 2-3 bar below the hydrogen equilibrium pressure to ˜1 bar, are to be used.

Of particular interest is the cpi diagram of the Ti doped Na₂ LiAlH₆ /(2 NaH+LiH+Al) system (FIG. 7, Example 3) which also has only one well pronounced pressure plateau at 211° C. which is shifted by about 20 bar towards lower pressure as compared to that of the Na₃ AlH₆ /(3 NaH+Al) system. The presence of only one pressure plateau different from that of Na₃ AlH₆ in the cpi diagram of Na₂ LiAlH₆ clearly demonstrates that this is an as yet unknown reversible metal hydride system, having a maximum H₂ storage capacity of 2.9% by weight (up to 2.7% by weight of H₂ achievable in practice), rather than a mixture of Na₃ AlH₆ and Li₃ AlH₆. In addition, it can be seen from this diagram that a well-aimed, "tailor-made" change of the reversible H₂ dissociation pressure, i.e., the thermodynamic properties of the present hydride system, is possible by a partial substitution of the sodium in Na₃ AlH₆ by lithium. Such well-aimed changes of the thermodynamic parameters by partial exchange of a metal component have been possible to date, in particular, with the reversible metal hydride system, LaNi₅ H₆ /LaNi₅. They are of technical importance, inter alia, due to the fact that the combination of two or more of such metal hydrides having different H₂ dissociation pressures is the basis for the function of metal hydride heat pumps (Sandrock 92, pages 234-237).

In addition, the cpi diagrams of all the three systems studied (FIGS. 6 and 7) exhibit two other features of these systems which are important in view of technical applications, namely the absence of hysteresis effects (the H₂ absorption curves are identical with those of H₂ desorption), and the almost horizontal course of the H₂ pressure plateaus. The absence of hysteresis effects means that no immanent losses of pressure and thus energy occur in the hydrogen charging and hydrogen decharging of these systems. The consequence of the horizontal course of the H₂ pressure plateaus is that hydrogen charging and hydrogen decharging can proceed at a constant hydrogen pressure in the gas volume when the hydride bed is at a constant temperature.

The dependence of the H₂ dissociation pressure on the temperature of the Ti doped NaAlH₄ (Equation 4) and Na₃ AlH₆ (Equation 7) systems was experimentally established using the cpi diagrams at 180 and 211° C. (Examples 1 and 4). By reason of the H₂ dissociation pressures, the first dissociation step of the Ti doped NaAlH₄ system is to be classified as a so-called low temperature hydride system, and the second as a medium temperature hydride system (Buchner, 1982, pages 26-29). Thus, the two-step reversible Ti doped metal hydride system NaAlH₄ /(NaH+Al) (Equation 6) consists of a low temperature and a medium temperature hydride step. The present invention for the first time provides reversible low and medium temperature hydride systems based on the light metals Na, Li and Al. Their reversible H₂ capacities are theoretically and practically higher than those of the as yet known low and medium temperature hydrides (cf. page 2).

The reversible alkali metal alanates according to the invention are suitable as hydrogen storage systems for mobile and stationary use. Their technical advantages as compared to high temperature hydrides, such as MgH₂, are their substantially reduced operating temperatures (e.g., 150° C. instead of ≧300° C.), and as compared to low temperature hydrides, their higher H₂ storage capacities and lower estimated material costs. Due to the relatively low reaction enthalpy of the alkali metal alanates (see above) and their low operating temperatures, it is considered that, when used as H₂ storage materials for, e.g., fuel cells or combustion engines, the hydrogen consumer can supply enough waste heat on a temperature level required for the desorption of the hydrogen from the alanate. Thus, for example, the operating temperature of the phosphoric acid fuel cell, i.e., 160° C., is within this temperature range (cf. J. Bentley et al., Proc. Intersoc. Energy Convers. Eng. Conf. 1994, 29th, 1103). Another advantage for driving fuel cells is the high purity of the hydrogen desorbed from the alanate, such as, in particular, the absence of carbon monoxide.

For increasing the total energy density, alkali metal alanates as H₂ storage materials can be combined with magnesium hydride storage materials in a number of different ways. In addition, they may serve, if appropriate, as intermediate H₂ storage materials in the MgH₂ /Mg based high temperature heat storage (cf. A. Ritter, VGB Kraftwerkstechnik (Engl. ed.) 1992, 72, 311).

The invention is further illustrated by the following Examples without being limited thereto. All experiments with air-sensitive substances were performed under a protective atmosphere, e.g., argon. The solvents employed were free from air and water.

EXAMPLE 1

(Na₃ AlH₆ and β-TiCl₃ doped Na₃ AlH₆ as reversible H₂ storage materials)

Na₃ AlH₆ was prepared from NaAlH₄ and NaH in heptane by the method of Zakharkin et al. (Dokl. Akad. Nauk SSSR, Engl. ed. 1962, 145, 656). Commercially available NaAlH₄ was purified by dissolving in THF and precipitating with ether (Clasen, Angew. Chem. 1961, 73, 322). After drying in vacuo, the crystalline NaAlH₄ obtained showed very broad hydride bands in the infrared (IR) spectrum (KBr) in the region around 720, 900 and 1670 cm⁻¹ ; bands from complexed THF or ether are not present in the spectrum. Elemental analysis (calculated values for NaAlH₄): Na 42.71 (42.75); Al 49.46 (49.96); H 7.62 (7.47); C 0.28 (0.0) %. The alcoholysis of NaAlH₄ yielded 99.3% of the calculated quantity of hydrogen.

16.57 g (0.31 mol) of the purified NaAlH₄ and 14.87 g (0.62 mol) of NaH (Fluka) were suspended in 120 ml of n-heptane, and the suspension was intensely stirred in an autoclave at an H₂ pressure of 140 bar and at 162° C. (inside temperature) for 72 h. Na₃ AlH₆ was separated from the solvent by filtration, washed with pentane and dried in vacuo to obtain 30.90 g of a fine light grey powder. Na₃ AlH₆ was identified by X-ray powder diffraction analysis and IR spectroscopy (KBr; very broad bands at 500-1000 and around 1300 cm⁻¹ ; the band at ˜1700 cm⁻¹, see above, is absent) Elemental analysis of Na₃ AlH₆ (calculated values): Na 67.27 (67.62); Al 26.15 (26.45); H 5.84 (5.93); C 0.88 (0.0) %. The thermovolumetric analysis of an ˜1 g sample (4° C./min up to 270° C.; Chem. Ing. Tech. 1983, 55, 156) yielded 96% of the hydrogen quantity calculated for the dissociation to 3 NaH+Al (Equation 7).

For doping with titanium, 15.99 g (157 mmol) of Na₃ AlH₆ was mixed with 0.48 g (3.1 mol) of β-TiCl₃, and 30 ml of ether was added thereto. The stirred suspension immediately adopted a deep brown colour, and H₂ evolution started. When the H₂ evolution was complete (40 min), the stirred suspension had liberated 110 ml (4.6 mmol) of H₂. The ether was evaporated in vacuo, and the residue was dried in vacuo to obtain 16.46 g of Ti doped Na₃ AlH₆ as a brown, air-sensitive powder the IR spectrum of which corresponded to that of Na₃ AlH₆ (see above). Elemental analysis (calculated values): Na 65.92 (65.63); Al 24.75 (25.68); H 5.28 (5.76); Ti 1.28 (0.91); Cl 1.86 (2.02); C 0.74 (0.0) %. Thermovolumetric analyses (see above) performed up to 270° C. and 500° C. yielded 97% and 98%, respectively, of the hydrogen quantity calculated for the dissociation to 3 NaH+Al and to 3 Na+Al, respectively. The thermovolumetric curve of Ti doped Na₃ AlH₆ to 3 NaH +Al is shifted by about 50° C. towards lower temperatures as compared to that of pure Na₃ AlH₆.

In order to test their suitability as reversible H₂ storage materials, 2.6 g samples each of pure and Ti doped Na₃ AlH₆ were subjected to a number of dehydrogenation/rehydrogenation cycles (cycle tests) under the same conditions. The cycle tests in this example were performed in a so-called open system, i.e., fresh hydrogen (technical hydrogen, 99.9%) was taken from a hydrogen pressure tank in each hydrogenation, and hydrogen was desorbed against normal pressure in each dehydrogenation.

Dehydrogenation: The sample is heated at 4° C./min from room temperature to 270° C., and then the temperature is kept constant until the H₂ evolution is complete; the time course of H₂ evolution together with the inside temperature of the sample can be recorded with the aid of an automatic gas burette (Chem. Ing. Tech. 1983). The hydrogenation is performed for 51/2 h at 200° C. while the H₂ pressure in the autoclave decreases from 60 to ˜40 bar.

The dependence of hydrogen storage capacity (measured through the quantity of hydrogen released during the dehydrogenation) on the number of cycles of pure and Ti doped Na₃ AlH₆ is shown in FIG. 3. Under the stated conditions, the reversible H₂ content of the Ti doped Na₃ AlH₆ /(3 NaH+Al) system is from 2.1 to 2.5% by weight (theoretical H₂ content: 2.84% by weight) which is significantly higher than that of undoped Na₃ AlH₆. In addition, the Ti doped Na₃ AlH₆ exhibits a considerably better cycle stability than pure Na₃ AlH₆.

EXAMPLE 2

(pure and Ti(OBu)₄ doped Na₃ AlH₆ as reversible H₂ storage materials; rate of H₂ desorption as a function of temperature; 100 cycle test)

9.58 g (94 mmol) of Na₃ AlH₆ (Example 1) was suspended in 30 ml of ether, and 0.64 ml (1.9 mmol, 2 mole %) of titanium tetra-n-butylate (Ti(OBu)₄) was added to the suspension with stirring (with a syringe through a septum). The amount of H₂ evolved (cf. Example 1) was 93 ml (2.1 H₂ /Ti). After evaporating the ether in vacuo, 10.13 g of Ti doped Na₃ AlH₆ remained.

For characterizing their usefulness as reversible H₂ storage materials, the rates of H₂ desorption of pure and of Ti doped Na₃ AlH₆ were measured at temperatures of 140, 160, 180 and 200° C. To this end, 1.75 g each of the alanate samples contained in glass vessels were placed in an oven preheated to the respective temperature, and the time course of the H₂ evolution was recorded with the aid of an automatic gas burette connected with the glass vessel (Chem. Ing. Tech. 1983; see FIG. 1). As can be seen from FIG. 1, the Ti doping causes a dramatic improvement of the H₂ desorptivity of Na₃ AlH₆.

Another sample (7.41 g) of the Na₃ AlH₆ doped with 2 mole % of Ti(OBU)₄ (see above) was subjected to 100 cycles of a dehydrogenation/rehydrogenation test in a closed system. The sample (which had been preliminarily pressed into tablets of about 1.0 g/ml) was placed in a 45 ml autoclave which was connected to a 100 ml pressure tank via a capillary. At specified time intervals, the autoclave was alternately heated at 230° C. for 11/4 h for dehydrogenation and maintained at 170° C. for varying periods of time for rehydrogenation. The variation of H₂ pressure in the system in the range between 30 and 42 bar was recorded on a two-channel plotter with the aid of a pressure/voltage converter together with the temperature of the autoclave. Through the pressure variation in the system, the reversible H₂ capacity of the sample could be determined to be 1.64-1.83 and 1.79-2.06% by weight in the 100 cycle test for hydrogenation times of 11/4 and 41/2 h, respectively.

Example 3

(β-TiCl₃ doped Na₂ LiAlH₆ as a reversible H₂ storage material)

Na₂ LiAlH₆ was prepared by reacting NaAlH₄ with NaH and LiH in a molar ratio of 1:1:1 in n-heptane. From 6.79 g (126 mmol) of NaAlH₄, 3.04 g (127 mmol) of NaH and 0.97 g (122 mmol) of LiH in 90 ml of n-heptane, there was obtained 11.08 g of Na₂ LiAlH₆ as a fine light grey powder in analogy to Example 1. The IR spectrum of the Na₂ LiAlH₆ corresponded to that of Na₃ AlH₆ (Example 1; there were no IR spectroscopic indications of NaH, LiH or NaAlH₄). Elemental analysis (calculated values for Na₂ LiAlH₆): Na 53.98 (53.50); Al 29.87 (31.39); Li 7.88 (8.08); H 6.50 (7.04); C 1.56 (0.0) %. A thermovolumetric analysis (cf. Example 1) performed up to 500° C. yielded 98% of the hydrogen quantity calculated for the dissociation to 2 Na+LiH+Al.

5.87 g (68 mmol) of Na₂ LiAlH₆ was doped with 2 mole % (1.4 mmol, 0.22 g) of β-TiCl₃ in ether as described in Example 1. The amount of H₂ evolved upon doping was 2.1 mmol. Elemental analysis of the 6.03 g of Ti doped Na₂ LiAlH₆ obtained (calculated values in parentheses): Na 51.06 (51.64); Al 30.17 (30.30); Li 7.59 (7:80); H 5.96 (6.79); Ti 1.05 (1.08); Cl 2.46 (2.39); C 1.71 (0.0) %. The cpi. diagram of the Ti doped Na₂ LiAlH₆ at 211° C. is shown in FIG. 7. Ti doped Na₂ LiAlH₆ was subjected to a 28 cycle test under the same conditions as those used in Example 1. As shown in FIG. 9, the reversible H₂ content of this system is between 2.10 and 2.51% by weight. With a hydrogenation time of 16 h, an H₂ capacity of up to 2.7% by weight can be achieved.

EXAMPLE 4

(NaAlH₄ and β-TiCl₃ doped NaAlH₄ as reversible H₂ storage materials)

26.83 g (0.50 mol) of the purified NaAlH₄ (Example 1) was doped with 2 mole % (10.2 mmol, 1.58 g) of β-TiCl₃ in 150 ml of ether as described in Example 1. The amount of H₂ evolved upon doping was 14.6 mmol, from which a reduction of titanium to the zero-valent state can be concluded. Elemental analysis of the 28.33 g of Ti doped NaAlH₄ obtained (calculated values): Na 41.80 (40.27); Al 46.81 (47.26); H 6.95 (7.06); Ti 1.46 (1.68); Cl 2.79 (3.73); C 0.20 (0.0) %. The IR spectrum of the Ti doped NaAlH₄ corresponded to that of pure NaAlH₄ (Example 1). Thermovolumetric analyses (cf. Example 1; 4° C./min) performed up to 200, 270 and 500° C. yielded 104, 96% and 97%, respectively, of the hydrogen quantity calculated for the dissociation to 1/3 Na₃ AlH₆ +2/3Al (detected by IR and X-ray powder diffraction analysis), NaH+Al (X-ray powder diffraction analysis) and Na+Al, respectively. The thermovolumetric curve of Ti doped NaAlH₄ up to 200° C. is shifted by 85° C. towards lower temperatures as compared to that of pure NaAlH₄.

The course of the cycle tests, performed on samples (2.4 g) of pure and of Ti doped NaAlH₄ under different hydrogenation conditions (dehydrogenation performed as in Example 1), is shown in FIG. 4. The cpi diagram of Ti doped NaAlH₄ is shown in FIG. 6.

EXAMPLE 5

(Ti(OBu)₄ doped NaAlH₄ as a reversible H₂ storage material)

The doping of NaAlH₄ with Ti(OBu)₄ in ether was performed in analogy to Example 2. There was employed 10.96 9 (203 mmol) of purified NaAlH₄ (Example 1), 25 ml of ether, and 1.39 ml of Ti(OBu)₄ (2 mole %). The amount of hydrogen evolved was 205 ml (2.1 H₂ /Ti). After drying in vacuo, 12.40 g of the Ti doped NaAlH₄ was obtained. The determination of the rate of H₂ desorption on samples (1.35 9) of the Ti doped and undoped NaAlH₄ at different temperatures was performed as in Example 2. The measuring results (FIG. 2) show, inter alia, that the Ti doped NaAlH₄ supplies 4.5% by weight of H₂ within a few hours at 160° C. already.

Another sample of purified NaAlH₄ (2.42 g, 44.8 mmol) was doped with 2 mole % of Ti(OBu)₄ as described in Example 2, but using pentane (10 ml) as the solvent instead of ether. After stirring the mixture at room temperature for one hour, the evolution of 42 ml of gas was observed. After evaporating the solvent and drying the residue in vacuo, 2.61 g of Ti doped NaAlH₄ remained in the form of a brown powder. When thermolysed (up to 270° C., cf. Example 1), it yielded 1.56 1 of H₂ (20° C./1 bar), corresponding to 5.0% by weight of H₂. The course of the rehydrogenation of the solid thus obtained at 170° C./152 bar of H₂ (initial pressure) in comparison with the rehydrogenation of the correspondingly thermolyzed samples of NaAlH₄ doped with 2 mole % of β-TiCl₃ and of undoped NaAlH₄ (Example 4) is shown in FIG. 5. After 15 h under the stated conditions, the sample doped with Ti(OBu) 4 achieved a degree of rehydrogenation of 78% (3.9% by weight of H₂). The corresponding values for the β-TiCl₃ doped and the undoped samples are 78% (3.9) and 15% (0.8%), respectively.

EXAMPLE 6

(Ti(OBu)₄ doped NaAlH₄ as a reversible H₂ storage material; doping without a solvent)

2.34 g (43.3 mmol) of the purified NaAlH₄ (Example 1) in solid form was whirled up with a magnetic stirring bar, and 0.30 ml (0.88 mmol) of titanium tetrabutylate was added with a syringe through a septum. The initially white sodium alanate was turned light brown thereby, and evolution of 24 ml of hydrogen (=2.3 H/Ti) occurred within 40 min. Subsequently, 2.49 g of this material was employed as a reversible hydrogen storage material. Thermolysis up to 270° C. (cf. Example 1) yielded 1.46 1 of H₂ (20° C./1 bar), corresponding to 4.9% by weight. The residue was rehydrogenated at 170° C. and between 143 and 120 bar within 15 h and again subjected to thermolysis as above. The reversible H₂ content was 3.6% by weight, corresponding to a degree of rehydrogenation of 74%.

EXAMPLES 7-25

1.3 g portions of the purified NaAlH₄ (Example 1) were each suspended in 20 ml of ether, and to the stirred suspension was added 5 mole % (based on NaAlH₄) of the respective metal compound. After 20-60 min (completion of the H₂ evolution), the solvent was evaporated, and the residues dried in vacuo. They were subjected to a thermolysis up to 270° C. as described in Example 1, and the H₂ volumes evolved were determined (Table 1, column "1st thermolysis"). The solids were then hydrogenated in an autoclave at 120° C. and 150 bar (initial pressure) to a minimum of 130 bar of H₂ pressure for 24 h, and subsequently again thermolyzed up to 270° C. The ratios of the H₂ volumes of the 2nd to those of the 1st thermolyses (in %) yield the degrees of rehydrogenation stated in Table 1.

EXAMPLE 26

(Ti(OBu)₄ and LaNi₅ doped NaAlH₄ as a reversible H₂ storage material)

A sample of the purified (Example 1) NaAlH₄ (1.87 g, 34.6 mmol) in solid form was stirred with 380 mg (17% by weight) of LaNi₅ powder (Alfa, 99.5%) and then doped with 2 mole % of Ti(OBu)₄ in 20 ml of ether as described in Example 2. The amount of hydrogen evolved was 34.6 ml (2.1 H₂ /Ti). After evaporating the ether and drying in vacuo, 2.48 g of the LaNi₅ and Ti(OBu)₄ doped NaAlH₄ was obtained. Thermolysis up to 270° C. (as in Example 1) yielded 4.1% by weight of H₂. After hydrogenating the dehydrogenated sample (120° C./110-90 bar of H₂ /24 h), an H₂ content of 3.1% by weight was found upon renewed thermolysis up to 270° C., corresponding to a degree of rehydrogenation of 76%. In comparison, a sample of NaAlH₄ which was doped with Ti(OBu)₄ only (Example 5) showed a degree of rehydrogenation of only 60% under the same conditions.

                  TABLE 1                                                          ______________________________________                                         Degrees of rehydrogenation.sup.a) of                                           dehydrogenated NaAlH.sub.4 as a function of the dopant                                         1st thermo- 2nd thermo-                                                                             degree of                                 Ex.             lysis.sup.c) [% by                                                                         lysis.sup.c) [% by                                                                      rehydro-                                  No.  dopant.sup.b)                                                                             weight of H.sub.2)                                                                         weight of H.sub.2)                                                                      genation [%]                              ______________________________________                                          7   --         5.52        0.55     10                                         8   TiCl.sub.4 4.51        2.85     63                                         9   β-TiCl.sub.3                                                                         4.75        2.96     62                                        10   HTiCl.0.5THF                                                                              5.00        3.07     61                                        11   Ti(OBu).sub.4                                                                             4.23        2.60     61                                        12   Cp.sub.2 TiCl.sub.2                                                                       4.48        2.34     52                                        13   ZrCl.sub.4 4.71        2.59     55                                        14   Cp.sub.2 ZrCl.sub.2                                                                       4.40        2.89     66                                        15   VCl.sub.3  4.81        2.65     55                                        16   Cp.sub.2 VCl.sub.2                                                                        4.47        2.11     47                                        17   NbCl.sub.3 4.59        1.91     42                                        18   YCl.sub.3  4.59        2.20     48                                        19   LaCl.sub.3 4.56        2.62     57                                        20   CeCl.sub.3 4.53        2.47     54                                        21   PrCl.sub.3 4.51        2.64     59                                        22   NdCl.sub.3 4.54        3.10     68                                        23   SmCl.sub.3 4.42        2.77     63                                        24   FeCl.sub.2 4.65        2.13     46                                        25   NiCl.sub.2 :1.5THF                                                                        4.69        2.24     48                                        ______________________________________                                          .sup.a) hydrogenation conditions: 120° C./150 130 bar of H.sub.2        /24 h.                                                                         .sup.b) 5 mole % each, based on NaAlH.sub.4.                                   .sup.c) form room temperature to 270° C. at 4° C./min; then      270° C. until the H.sub.2 evolution was completed.                

EXAMPLE 27

(TiCl₄ doped KAlH₄ as a reversible H₂ storage material)

2.46 g of KAlH₄ (35.1 mmol) was suspended in 20 ml of ether, and 0.1 ml of TiCl₄ (0.91 mmol=2.6 mole %) of TiCl₄ was added to the stirred suspension; spontaneous gas evolution occurred. The amount of hydrogen evolved was about 200 ml. After evaporating the ether in vacuo, 2.65 g of the TiCl₄ doped KAlH₄ was obtained in the form of a black powder. Thermolysis up to 320° C. yielded 2.4% by weight of H₂. After hydrogenating the dehydrogenated sample (140° C./150-140 bar/18 h), an H₂ content of 0.8% by weight was found upon renewed thermolysis up to 320° C., corresponding to a degree of rehydrogenation of 32.5%. 

We claim:
 1. A process for the reversible storage of hydrogen, said process comprising:a) dehydrogenating a complex alkali metal aluminum hydride of the formula (1):

    M.sup.1.sub.p(1-x) M.sup.2.sub.px AlH.sub.3+p              ( 1)

wherein M¹ is selected from the group consisting of Na and K; M² is selected from the group consisting of Li and K; 0≦x≦0.8; and 1≦p≦3; to yield hydrogen gas and at least one other product; and b) hydrogenating said other product to yield said complex alkali metal aluminum hydride of the formula (1);wherein steps a) and b) are carried out in any order.
 2. The process according to claim 1, wherein steps a) and b) are catalyzed.
 3. The process according to claim 2, wherein steps a) and b) are catalyzed by a process comprising doping the alkali metal aluminum hydride of the formula (1) by reacting or stirring the alkali metal aluminum hydride of the formula (1) with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 4. The process according to claim 3, wherein said dopant is selected from the group consisting of alcoholates, halides, hydrides and organometallic and intermetallic compounds of said metal or combination of metals.
 5. The process according to claim 3, wherein the doping is performed in an organic solvent.
 6. The process according to claim 3, wherein the doping is performed in the absence of a solvent.
 7. The process according to claim 3, wherein the doping is performed using 0.2 to 10 mole % of the dopant, based on the alkali metal aluminum hydride.
 8. The process according to claim 7, wherein the doping is performed using 1 to 5 mole % of the dopant, based on the alkali metal aluminum hydride.
 9. The process according to claim 1, wherein steps a) and b) are performed at a temperature greater than 100° C., but less than or equal to 300° C.
 10. The process according to claim 1, wherein step b) is performed at a hydrogen pressure ranging from 0.1 to 100 bar above the hydrogen equilibrium pressure at the temperature at which step b) is performed.
 11. The process according to claim 10, wherein step b) is performed at a hydrogen pressure ranging from 2 to 50 bar above the hydrogen equilibrium pressure at the temperature at which step b) is performed.
 12. The process according to claim 1, wherein step a) is performed at a hydrogen pressure ranging from 0.1 bar below the hydrogen equilibrium pressure at the temperature at which step a) is performed to 0.1 bar.
 13. The process according to claim 1, wherein step a) is performed at a hydrogen pressure ranging from 2-3 bar below the hydrogen equilibrium pressure at the temperature at which step a) is performed to 1 bar.
 14. The process according to claim 1, wherein the alkali metal aluminum hydride of formula (1) is Na₂ LiAlH₆.
 15. The process according to claim 14, which comprises doping the Na₂ LiAlH₆ with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 16. The process according to claim 1, wherein the alkali metal aluminum hydride of formula (1) is NaAlH₄.
 17. The process according to claim 16, which comprises doping the NaAlH₄ with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 18. The process according to claim 1, wherein the alkali metal aluminum hydride of formula (1) is Na₃ AlH₆.
 19. The process according to claim 18, which comprises doping the Na₃ AlH₆ with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 20. A composition of matter selected from the group consisting of:a) Na₂ LiAlH₆ ; b) Na₂ LiAlH₆ doped with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals; c) NaAlH₄ doped with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals; and d) Na₃ AlH₆ doped with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 21. The composition of matter according to claim 20, which is Na₂ LiAlH₆.
 22. The composition of matter according to claim 20, which is Na₂ LiAlH₆ doped with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 23. The composition of matter according to claim 20, which is NaAlH₄ doped with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals.
 24. The composition of matter according to claim 20, which is Na₃ AlH₆ doped with a dopant comprising a metal selected from the group consisting of transition metals of Groups III to V of the periodic table, iron, nickel and the rare earth metals, or a combination of said metals. 